1. Field of the Invention
The present invention generally relates to signal transmitting and receiving apparatus. More particularly, the present invention is directed to improvements in a signal transmitting and receiving apparatus, such as a digital cordless telephone apparatus, which is operative to perform selectively a signal transmitting operation in which a transmissible digital phase-modulated signal is produced based on transmissible digital data and the transmissible phase-modulated signal is frequency-converted to be allocated in a higher frequency band with a predetermined carrier signal so as to be transmitted and a signal receiving operation in which a received digital phase-modulated signal is frequency-converted to be allocated in a lower frequency band with the predetermined carrier used in common to the signal transmitting operation and the signal receiving operation and received digital data are obtained based on the received digital phase-modulated signal having been frequency-converted.
2. Description of the Prior Art
There has been a proposal to adopt a time sharing multiplex communication system referred to as a TDD system or a TDMA system for communication between two digital cordless telephone apparatus, in which each of the digital cordless telephone apparatus is operative to perform alternately a signal transmitting operation and a signal receiving operation during the communication. The digital cordless telephone apparatus, by which the communication with the other is carried out in accordance with the time sharing multiplex communication system, is provided with a transmitting circuit block in which a digital phase-modulated signal is produced through phase-modulation effected with transmissible digital data in accordance with the quadrature phase shift keying and then frequency-converted into a transmissible digital phase-modulated signal allocated in a higher frequency band and the transmissible digital phase-modulated signal is transmitted therefrom, and also provided with a receiving circuit block in which a received digital phase-modulated signal is frequency-converted to be allocated in a lower frequency band and received digital data are obtained by phase-demodulating the received digital phase-modulated signal frequency-converted after reception.
In the phase-modulation effected in accordance with the quadrature phase shift keying in the transmitting circuit block, a couple of carrier signals for modulation having the same frequency and a relative phase angle difference of ninety degrees are provided and the transmissible digital data are reformed into a couple of parallel data by a parallel data generator. Then, one of the carrier signal is subjected to balanced-modulation with one of the parallel data to produce a first modulated signal and the other of the carrier signal is subjected to balanced-modulation with the other of the parallel data to produce a second modulated signal, so that there is a phase angle difference of ninety degrees between the first modulated signal and the second modulated signal. Accordingly, the first and second modulated signals constitute a pair of quadrature modulated signals and are mixed with each other to produce the digital phase-modulated signal obtained through the phase-modulation according to the quadrature phase shift keying. Further, in the phase-demodulation effected to the received digital phase-modulated signal having been frequency-converted in the receiving circuit block, the received digital phase-modulated signal is subjected to both phase-demodulations with a couple of carrier signals for demodulation having the same frequency and a relative phase angle difference of ninety degrees, respectively, so as to produce first and second demodulated signals. The first and second demodulated signals are mixed with each other to produce the received digital data.
The transmissible digital phase-modulated signal which is formed by frequency-converting the digital phase-modulated signal obtained through the phase-modulation according to the quadrature phase shift keying to be allocated in the higher frequency band, has the carrier frequency the same as the carrier frequency of the received digital phase-modulated signal. Then, for the purpose of simplification in circuit configuration and reduction in number of circuit parts and elements, a carrier signal for frequency-conversion used for frequency-converting the digital phase-modulated signal, which is obtained through the phase-modulation according to the quadrature phase shift keying, into the transmissible digital phase-modulated signal allocated in the higher frequency band and another carrier signal for frequency-conversion used for frequency-converting the received digital phase-modulated signal to be allocated in the lower frequency band are arranged to be generated by a single carrier signal generator provided in common to the transmitting circuit block and the receiving circuit block.
When digital data are transmitted and received in such a manner as mentioned above under a situation wherein each of the transmissible digital phase-modulated signal and the received digital phase-modulated signal has the same carrier frequency, and the carrier signal for frequency-conversion used for frequency-converting the digital phase-modulated signal into the transmissible digital phase-modulated signal allocated in the higher frequency band and the carrier signal for frequency-conversion used for frequency-converting the received digital phase-modulated signal to be allocated in the lower frequency band are arranged to be generated by the carrier signal generator provided in common to the transmitting circuit block and the receiving circuit block, the carrier frequency of the digital phase-modulated signal which is frequency-converted to be allocated in the higher frequency band in order to produce the transmissible digital phase-modulated signal is equal to the carrier frequency of the received digital phase-modulated signal frequency-converted to be the lower frequency band. Therefore, the frequency of each of the carrier signals for modulation having the same frequency and the relative phase angle difference of ninety degrees and used for the phase-modulation which is carried out in accordance with the quadrature phase shift keying to produce the digital phase-modulated signal is equal to the carrier frequency of the received digital phase-modulated signal frequency-converted to be allocated in the lower frequency band. This results in such a fear that each of the carrier signals for modulation which are generated to be used for the phase-modulation according to the quadrature phase shift keying in the transmitting circuit block is undesirably mixed up in the received digital phase-modulated signal frequency-converted to be allocated in the lower frequency band so as to raise noise obstructions when the signal receiving operation is performed in the receiving circuit block.
Accordingly, to avoid the undesirable noise obstructions thus brought about, it is required to prevent the carrier signals for modulation which are generated to be used for the phase-modulation according to the quadrature phase shift keying from being supplied to a modulating portion of the transmitting circuit block, in which the phase-modulation according to the quadrature phase shift keying is carried out, when the signal receiving operation is performed in the receiving circuit block.
A carrier signal generating portion of the transmitting circuit block for generating the carrier signals for modulation having the same frequency and the relative phase angle difference of ninety degrees is generally constituted to form a phase-locked loop configuration. In the carrier signal generating portion constituted to form the phase-locked loop configuration, a stabilized carrier signal oscillator which operates with oscillation frequency selected to coincide with the frequency of each of the carrier signals for modulation is provided and an output of the stabilized carrier signal oscillator is directly derived to serve as one of the carrier signals for modulation and derived through a phase-shifter for providing phase shift of ninety degrees to serve as the other of the carrier signals for modulation. Accordingly, by controlling the stabilized carrier signal oscillator to be operative during a period wherein the signal transmitting operation is performed in the transmitting circuit block and to be inoperative during a period wherein the signal receiving operation is performed in the receiving circuit block, the carrier signals for modulation can be prevented from being supplied to the modulating portion of the transmitting circuit block when the signal receiving operation is performed in the receiving circuit block.
However, although it is necessary for putting such control of the stabilized carrier signal oscillator as mentioned above into practice to change the stabilized carrier signal oscillator from operative to inoperative or from inoperative to operative within an extremely short period, it is quite difficult in practice to cause the stabilized carrier signal oscillator to have so quick changes in its condition as required. Consequently, in case of previously proposed digital cordless telephone apparatus, the carrier signals for modulation which are generated to be used for the phase-modulation according to the quadrature phase shift keying in the transmitting circuit block are not effectively controlled to be prevented from being supplied to the modulating portion of the transmitting circuit block and therefore the noise obstructions caused in the received digital phase-modulated signal by the carrier signals for modulation mixed up in the received digital phase-modulated signal are not sufficiently suppressed.